Sensing Apparatus Having a Light Sensitive Detector Field

ABSTRACT

A sensing apparatus includes a sensor and a processor. The sensor includes at least one light sensitive detector. The processor determines a first control value to control a voltage differential across the at least one light sensitive detector, and compares the first control value with a reference value associated with a reference temperature. Based on the comparison, the processor provides adjustment information for adjusting at least one output of the sensing apparatus, and an operating parameter of the sensing apparatus other than the voltage differential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/051,833, filed on Feb. 24, 2016, which claims priority to UnitedKingdom Patent Application No. 1517267.9, filed on Sep. 30, 2015, bothof which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method and apparatus, and inparticular, but not exclusively, to a sensing apparatus having a lightsensitive detector.

BACKGROUND

A single photon avalanche detector (SPAD) is based on a p-n junctiondevice biased beyond its breakdown region. A high reverse bias voltagegenerates a sufficiently large electric field such that a single chargecarrier introduced into a depletion layer of the p-n junction device cancause a self-sustaining avalanche. The charge carrier may be released bythe impact of a photon (impact ionization). The SPAD may be quenched,allowing the device to be reset to detect further photons. The behaviorof the SPAD may vary depending on one or more factors such astemperature, aging and process variation.

SUMMARY

In one embodiment, a sensing apparatus comprises a sensor comprising atleast one light sensitive detector and a processor. The processor may beconfigured to determine a first control value which is used to control avoltage differential across the at least one detector, and compare thefirst control value with a reference value associated with a referencetemperature. Based on the comparison, the processor may be configured toprovide adjustment information for adjusting one or more outputs of thesensing apparatus, and an operating parameter different than the voltagedifferential, of the sensing apparatus.

The adjustment information may be dependent on the magnitude of thedifference between the first control value and the reference value, andwhether the first control value is greater or less than the referencevalue.

For a given magnitude of difference, a magnitude associated with theadjustment information may be different depending on whether the firstcontrol value is greater or less than the reference value.

For the given magnitude of difference the adjustment information mayrepresent a bigger adjustment if the difference between the firstcontrol value and the reference value is indicative that temperature isgreater than the reference temperature as compared to when thedifference between the first control value and the reference value isindicative that temperature is less than the reference temperature.

The operating parameter may comprise an oscillator frequency. Theoscillator frequency may be a clock signal frequency.

The adjustment information may be configured to adjust the clock signalfrequency such that the clock signal frequency is within a givenfrequency range such that one or more of harmonics of the clock signal,the clock signal frequency, one or more signals derived from the clocksignal, and harmonics of one or more signals derived from the clocksignal are outside one or more communication band frequencies. Theadjustment information may comprise trim information for trimming theoscillator frequency.

The sensing apparatus may be configured to determine a distance of anobject away from the apparatus, with the output of the sensing apparatusbeing the distance.

The sensing apparatus may comprise an array of light sensitivedetectors. At least one light sensitive detector may comprise a SPAD.

Another aspect is directed to a sensing method comprising determining afirst control value which is used to control a voltage differentialacross at least one light sensitive detector, and comparing the firstcontrol value with a reference value associated with a referencetemperature, Based on the comparison adjustment information may beprovided for adjusting one or more of an output of a sensing apparatusin which the at least one light sensitive detector is provided, and anoperating parameter different than the voltage differential, of thesensing apparatus. The method may further comprise adjusting one or moreof the outputs and the operating parameter using the adjustmentinformation.

The adjustment information may be dependent on the magnitude of thedifference between the first control value and the reference value, andwhether the first control value is greater or less than the referencevalue.

For a given magnitude of difference, a magnitude associated with theadjustment information may be different depending on whether the firstcontrol value is greater or less than the reference value.

For the given magnitude of difference the adjustment information mayrepresent a bigger adjustment if the difference between the firstcontrol value and the reference value is indicative that temperature isgreater than the reference temperature as compared to when thedifference between the first control value and the reference value isindicative that temperature is less than the reference temperature.

The operating parameter may comprise an oscillator frequency. Theoscillator frequency may be a clock signal frequency.

The adjustment information may be configured to adjust the clock signalfrequency such that the clock signal frequency is within a givenfrequency range such that one or more of harmonics of the clock signal,the clock signal frequency, one or more signals derived from the clocksignal, and harmonics of one or more signals derived from the clocksignal are outside one or more communication band frequencies.

The adjustment information may comprise trim information for trimmingthe oscillator frequency, and the method may comprise trimming theoscillator frequency using the trim information. The method may comprisedetermining a distance of an object away from the apparatus, with theoutput being the distance which is adjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example only, to the accompanyingdrawings in which:

FIG. 1 is a diagram of a SPAD with a quench and readout circuit;

FIG. 2 shows a schematic diagram of the voltage control used in someembodiments;

FIG. 3 shows schematically SPAD bias voltages and operating conditions;

FIG. 4 shows a block diagram of one embodiment;

FIG. 5 shows a method of one embodiment;

FIG. 6 schematically shows frequency harmonics of one embodiment andcommunication band frequencies;

FIG. 7 shows a first graph of ranging results for a set of devices;

FIG. 8 shows a second graph of ranging results for the set of devices towhich a first compensation method has been applied;

FIG. 9 shows a third graph of ranging results for the set of devices towhich a second compensation method has been applied; and

FIG. 10 shows a device having a SPAD arrangement.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Single-photon avalanche diodes (SPAD), are also called Geiger modeavalanche photo diodes (GAPD). These devices have a reverse biased p-njunction in which a photo-generated carrier can trigger an avalanchecurrent due to an impact ionization mechanism. SPADs may be designed tooperate with a reverse bias voltage well above the breakdown voltage.

FIG. 1 schematically shows a single photon avalanche diode (SPAD) 101.The SPAD 101 has a reverse biased p-n junction 102. The reverse biasedp-n junction 102 has a high reverse bias voltage (−V_(BREAKDOWN)). Withthe reverse bias voltage, a relatively high electric field is generatedsuch that a single charge carrier injected into the depletion layertriggers a self-sustaining avalanche via impact ionization. In otherwords, a photon impacting on the reverse biased p-n junction device 102releases a single charge which triggers a chain reaction releasing alarge number of electrons leading to a large current.

To reset the device 102, the current flow is quenched. Withoutquenching, the p-n junction device 102 may be permanently damaged.

Different types of quenching are known. For example, passive or activequenching may be used. Passive quenching may, for example, use aresistor in series with the SPAD. The avalanche current is effectivelyquenched as a voltage drop is developed across a relatively high valueresistance of the resistor. Alternatively, active quenching may be used.

FIG. 1 shows an example where passive quenching is used. A p-type MOSFET(metal-oxide-semiconductor field-effect transistor) 100 is provided inseries with the p-n junction device 102 and is connected between themore positive voltage V_(EXCESS) and the reverse biased p-n junctiondevice 102. A quenching voltage V_(QUENCH) is applied to the gate of theMOSFET 100. The MOSFET 100 acts as a relatively high resistanceresistor.

The voltage waveform at the node 106 between the MOSFET 100 and the p-njunction device 102 can be seen schematically in FIG. 1. Initially, theoutput of the node 106 is at a relatively high voltage. When the photonimpacts on the p-n device 102, this causes a relatively large current toflow rapidly which causes the voltage on node 106 to drop rapidly. Asthe quenching voltage is applied, the voltage at the node 106 rises backup to the initial voltage value. The voltage waveform at the node 106 ispassed through an inverter 104 to give a square waveform with the lowlevels of the wave representing the state prior to the impact of thephoton on the P-N device and after quenching, and the high levelrepresents the impact of a photon. The output of the inverter 104 can beprovided to detection circuitry to be processed. For example, the outputof the inverter 104 can be input to a counter which counts every timethe output of the inverter goes high.

It should be appreciated that the SPAD shown in FIG. 1 and the quenchingarrangement is by way of example only and other structures mayalternatively be used. For example, active quenching may be used. Otherpassive quenching arrangements may also be used.

In some embodiments, an array of SPADs is used. However, it should beappreciated that some embodiments may be used with a single SPAD.

A breakdown voltage is required to place the avalanche diode in theGeiger region of operation and cause the p-n device to operate as aSPAD. The breakdown voltage is controlled by the voltage differentialacross the p-n device rather than the absolute voltage values on eitherside of the reverse biased p-n junction device.

The breakdown voltage of the diode may be sensitive to one or more ofthe following factors: process variation, the SPAD design, variation inthe components over time, and temperature variation.

In a SPAD provided on a chip, on a die, or as part of a chip set, thisvoltage provided to the SPAD may be provided by a voltage source on thesame chip or die, or on another chip of the chip set or another die(packaged for example with the die having the SPAD). Alternatively, thevoltage supply may be an external supply. If a SPAD is reverse biasedwith a voltage differential (controlled by the voltage supply) which iseither too low or too high, the p-n device will not operate as a singlephoton sensitive avalanche diode.

In order to ensure correct operation of the SPAD taking into account oneor more of the above factors, the voltage used should be calibrated.Various different factors may be considered in determining an optimumbreakdown voltage for a particular SPAD or use of that SPAD. Forexample, in some scenarios, the optimum bias voltage may be consideredto be the voltage at which the SPAD achieves its maximum count rate orbest timing performance. Alternatively, in some embodiments, the optimumvoltage may be considered to be the middle of the region of operationwhich may provide a compromise between two or more of the count rate,timing performance and dark count rate. The SPAD may be sensitive tothermally generated carriers which fire the avalanche process. Theaverage number of counts per second when the SPAD is in completedarkness is referred to as the dark count rate and is a parameter whichis used in defining the detector noise. The reciprocal of the dark countrate defines the average time that the SPAD will remain biased abovebreakdown before being triggered by an undesired thermal process.Usually, a SPAD is designed so as to remain biased above breakdown for asufficiently long time in order to work correctly as a single photondetector.

As temperature conditions change, the breakdown voltage of the SPADdiode may drift. If the voltage provided by, for example, an on chipvoltage source is fixed, then the breakdown voltage may be insufficientor too high for the SPAD to operate in the Geiger mode and have theeffect as described previously.

In some embodiments, a SPAD arrangement may be required to operate overa wide range of temperature conditions. In some SPADs there may be analteration in breakdown voltage of around 0.1V per 10 degreescentigrade. This may be significant in that in some cases the operationregion of a SPAD may have an extent of 1 to 2 V between the lowestusable voltage and the highest usable voltage.

Reference is made to FIG. 2 which shows one embodiment. The arrangementcomprises a voltage source 4. This voltage source may provide, forexample, V_(EXCESS), as shown in FIG. 1. This voltage source may be anon chip voltage source, or the voltage source may be external to thechip. The voltage source may take any suitable format.

The voltage source 4 is configured to control the voltage applied to theSPAD array 2. In one embodiment, the voltage source may be used tocontrol V_(EXCESS) while −V_(BREAKDOWN) is kept constant. Alternatively,V_(EXCESS) may be kept constant and −V_(BREAKDOWN) may be controlled bythe voltage source. In some embodiments, both −V_(BREAKDOWN) andV_(EXCESS) may be varied.

In the following example, the voltage source will control V_(EXCESS).The voltage source 4 provides the voltage which is used in each SPAD ofthe SPAD array 2. Each SPAD may be as shown in FIG. 1. The output ofeach SPAD is provided to a digital counter 6. In one embodiment, thedigital counters 6 will count each time the output of the inverter 104goes high, as shown in FIG. 1. The output of the digital counter 6 isinput to a controller 8. The controller may take any suitable form andmay be implemented by hardware, software and/or a combination of thetwo. In some embodiments, the controller 8 may comprise a firmwarecontroller. In other embodiments, the controller 8 comprises at leastone processor. The output of the controller 8 is used to provide aninput to the voltage source 4. The input provided by the controller 8controls the voltage which is provided by the voltage source 4.

The controller 8 is configured to put the arrangement of FIG. 2 into acalibration mode. When the arrangement is in a calibration mode, acontrol signal is provided by the controller 8 to a light source driver9. In turn, the light source driver 9 will control a light source 12,switching it on or off. Where the device is part of a chip or the like,the light source 12 may be in the same package or a different package.

The controller 8 is configured to cause the voltage source to apply oneor more different voltages to the array. In some embodiments, thecontroller 8 may be configured to control the voltage source to cyclethrough a plurality of output voltage values. The counts associated witheach of the output calibration voltages are stored by the controller 8.Based on the results of the calibration, i.e. counts, the controllerwill select the appropriate voltage to be provided by the voltage sourceduring the normal operation of the SPAD array.

It should be appreciated that this is just one example of one way inwhich the voltage can be calibrated. Other embodiments may use any othersuitable method of calibration.

In some embodiments, the calibration cycle is carried out at definedtime intervals. In some embodiments, the normal operation mode may beinterspersed with calibration mode periods. In some embodiments,depending on the use of the SPAD arrangement, calibration may beperformed when the SPAD is not required to be in a normal mode ofoperation. In alternative embodiments, the calibration cycle iscontrolled to take place alternatively or additionally in response tothe determination of one or more conditions.

Reference is made to FIG. 3 which shows a summary of SPAD bias voltageand operating conditions. The voltage of the y-axis of FIG. 3 representsa magnitude value rather than an absolute value. If the breakdownvoltage, i.e., the voltage differential across the SPAD is insufficient,the SPAD never fires. This corresponds to reference 200. As the voltageincreases, the SPAD enters its normal operating range. This correspondsto reference 202. In this region the energy release by the impact of aphoton triggers the avalanche effect. As the voltage further increases,the voltage is too high for the SPAD to operate as required. This isregion 204. The divisions between the regions may not be as clearlydefined as shown in FIG. 3. Thus, in some embodiments, it may bedesirable for the SPAD voltage to be such that that the SPAD operateswell within the region 202 and not close to the border regions adjacenteither region 200 or region 204. Region 202 may be considered to be theregion in which the SPAD is the most photo responsive. Differentembodiments may use different options for selecting the most appropriatevoltage. It should be appreciated that in some embodiments, more thanone voltage setting option may be available.

In one embodiment, the voltage setting that provides the maximum SPADcurrent rate can be selected. The voltage setting can be increased froma low voltage, i.e., below the breakdown of the voltage of the SPAD)until the breakdown voltage of the SPAD is established. The breakdownvoltage of the SPAD 80 is determined when an appropriate count isdetected by the digital counter. A fixed voltage offset or an increasecan be applied so that that it is known how far beyond the breakdownvoltage the bias has been set. It should be appreciated that embodimentscan be used with any suitable method of calibrating the voltage.

The SPAD array used in different embodiments may have any suitableapplication. For example, some embodiments may be used in rangingapplications for determining how far away an object is or for detectingthe presence of an object within a given range or the like.

Reference is made to FIG. 4 which schematically shows an apparatusaccording to one embodiment. A laser diode 12 is controlled to emit apulse of light, as indicated by reference d. The light d from the laserdiode 12 is reflected from an object 420 and directed towards adetection array 418. The detection array comprises an array of SPADs,such as discussed previously. The light which is emitted from the laserdiode is also received by a reference array 416, without being reflectedby the target object. The outputs of the two arrays are provided torespective OR trees 412 and 414. In particular, the output of thereference array 416 is input to an OR tree array 412, the output ofwhich is provided to a time to digital converter 410. The output of thedetection array 418 is provided to the OR tree array 414, the output ofwhich is again input to the time to digital converter 410. Thus, an ORtree is provided for each array. The OR tree function is to allow theoutputs of the SPAD to be output in turn to the time to digitalconverter. Effectively, the input to the time to digital converter 410from the OR tree 412 represents time T_(o), i.e., the time at which thelight pulse emitted by the laser diode is detected by the referencearray. The output of the OR tree 414 provides the time equal toT_(o)+2xT_(d). T_(d) is the time taken for the pulse of light emitted bythe laser diode to be emitted, reflected from the target and detected bythe return array.

The output of the time to digital converter is input to a processor 408.The processor 408 is configured to provide an output 426 whichrepresents the distance of the target from the device, i.e., d. Thevalues of T_(o) and T_(o)+2xT_(d) are used to determine d in anysuitable manner.

The processor is also configured to provide an output to a firstregister controller 406, the output of which is provided to anoscillator 404. The output of the processor controls the output of theregister controller which in turn controls the frequency of theoscillator. In some embodiments, the output of the register controlleris used to trim or adjust the oscillator frequency.

In some embodiments the first register controller may be provided by alook up table. The output of the processor provides the look up value,the output which controls the trim value which is used to control thefrequency of the oscillator. This is described in more detail later.

The output of the oscillator 404 is used by a phase lock loop 402 whichmultiplies the clock signal by a factor. In this embodiment, the factoris 64. The output of the phase lock loop PLL 402 provides a clock ortiming signal which is used by various other blocks. The output of thephase lock loop 402 is provided to a divider 400. In this example thedivider 400 divides the frequency of the signal output by the PLL by afactor of 6. However, this is by way of example only and in otherembodiments, the divider may be omitted or divided by another number.The signal which is output by the divider is a clock signal and isprovided to the driver 9 for the laser diode 12. This is used to providea drive signal which drives the laser diode.

The output of the processor 408 is provided to a second registercontroller 424, the output of which is provided to a voltage controller422. The voltage controller 422 provides a control signal which isprovided to the reference array 416 and to the return array 418 tothereby control the voltage applied across each SPAD.

In some embodiments, the control is swept repeatedly during deviceoperation to ensure optimal SPAD firing conditions, compensating forprocess and temperature variations.

Any of the methods for voltage calibration previously described may beused. For example, the processor may be configured during calibration tocontrol the voltage which is applied to the SPAD. In one embodiment, thevoltage values are controlled to slowly increase until the SPADs beginto respond to incident value. The processor uses the values from thedigital counter to determine which SPADs are firing. Once the processorfinds the correct voltage (as the SPADs are firing), it will add acertain amount of bias to the voltage to ensure that the SPADs areoperation within their operating range.

Reference is made to FIG. 5 which is schematically shows a method of anembodiment. In step S1, the ranging operation is started to determinethe distance of an object from the array. In other words, a light signalwill be emitted by the laser diode and will be detected by therespective reference and the detector arrays. In some embodiments, thisranging operation is purely a test operation. In other embodiments, theranging operation may be an actual operation.

In step S2, a calibration operation is performed such as outlinedpreviously to determine a control value to be provided by the voltagecontroller 422 to control the SPAD voltage differential.

In step S3, the control value to be provided by the voltage controlleris compared to the default value. In some embodiments, the default valuemay be stored in a non-volatile memory. The default value may be a valuewhich is programmed at the time of manufacture for a given referencetemperature RT. In some embodiments the RT is 23° C.

It should be appreciated that the control and reference values which arecompared may be actual voltage values or values which are representativeof the values. For example, the control and reference values may berepresented by index values.

In step S4, any adjustment which is required is determined. In theembodiments, the adjustment may be to one or more of the oscillatorfrequency and the resultant distance reported to the user. Both theoscillator frequency and the distance reported to the user may besensitive to variations in temperature. The voltage values which causethe SPADs to fire vary with temperature. For the typical temperaturevalues over which the SPAD will operate (e.g. −20° C. to 60° C.) thevoltage required to get the SPADs to fire will linearly increase withtemperature. A comparison of the reference voltage control value and thedetermined voltage control value will give a measure of the temperatureand the result can be used to compensate one or more of the oscillatorfrequency and the determined distance. This step may be performed in theprocessor.

If the new voltage control value is less than the reference voltagecontrol value, then it can be determined that the temperature is colderthan the temperature associated with the reference value. The size ofthe difference represents the magnitude of the difference from thereference temperature. This information is used to determine, forexample, if the oscillator is to be trimmed and if so by how much.

If the new voltage control value is more than the reference voltagecontrol value, then it can be determined that the temperature is hotterthan the temperature associated with the reference value. Again, thesize of the difference represents the magnitude of the difference fromthe reference temperature. This information is used to determine, forexample, if the oscillator is to be trimmed and if so by how much.

If the new voltage control value is the same as the reference controlvalue, then it can be determined that the temperature is the same orsimilar to the reference value and no adjustment is required.

As mentioned, this comparison is used to determine, for example, if theoscillator is to be trimmed and if so by how much. This is explained inmore detail below.

Some embodiments are configured to adjust or trim the oscillatorfrequency to avoid interference. In particular, embodiments areconfigured to ensure that the harmonics of the oscillator frequencyavoid one or more mobile telecommunication frequency bands. For example,in some embodiments, the oscillator frequency is controlled to avoidharmonics with one or more of the so-called 4G frequency bands and oneor more GSM bands. For example, the frequency bands may be at one ormore of 800 MHz, 1800 MHz and 2.6 GHz for 4G and 850 MHz and 900 MHz forGSM.

In some embodiments, the frequency of the PLL may be 835.84 MHz and thedriver clock frequency may be 139.31 MHz (⅙ of the PLL frequency). Theoscillator frequency is 13.06 MHz ( 1/64 of the PLL frequency). The PLLfrequency and the driver clock frequency are derived from the oscillatorclock frequency. The harmonics of 139.31 MHz are shown in FIG. 6 alongwith the positions of the 4G band of 800 MHz, and the two GSM bands.When the oscillator, PLL and driver clock are operating at the correctfrequencies, then there is no interference with the respectivecommunication frequency bands. However, due to temperature changes, thefrequency of the oscillator, PLL and driver clock may drift andpotentially cause interference with the communication bands. Changes inthe oscillator frequency will lead to changes in the PLL frequency andthe driver clock frequency. Interference can arise from the PLLfrequency and/or from the harmonics of the driver clock frequency.

In some embodiments, it is desirable to have a guard band between thefrequency of harmonics/PLL frequency and the respective communicationsband. In one embodiment, a guard band of 40 kHz may be used. However,this is by way of example and in different embodiments, a differentguard band value may be used. In the example shown, with a guard band of40 KHz, this means that the oscillator or clock frequency needs to be ina window between 12.90 MHz to 13.26 MHz. It should be appreciated thatthese values are by way of example only and different embodiments may bearranged to be used with different device frequencies and/or to avoidone or more different frequency bands.

In some embodiments, the oscillator will be trimmed in order to keep theoscillator frequency in the above window. The oscillator is set as adefault to operate at a reference temperature of 23° C. The default orreference temperature value is by way of example only and may bedifferent in other embodiments. In some embodiments, the behavior of theoscillator may not be symmetric. For example, one third of the driftfrom the ideal frequency occurs when the temperature is lower than thereference temperature and two thirds of the drift from the idealfrequency occurs when the temperature is higher than the referencetemperature. In other words, the drift from the ideal frequency isgreater when the temperature is higher than the reference temperature ascompared to when the temperature is lower than the referencetemperature. Accordingly, in this example, the nominal frequency couldbe set to be 13.02 MHz in the case of the above example window. Thisallows for a lower drift of 0.65% (when the temperature falls below thereference temperature) and an upper drift of 1.57% (when the temperaturerises above the reference temperature).

In some embodiments, the second register controller 424 is a look uptable which is response to an output provides a control value which isused by the voltage controller to control the voltage differentialacross the SPADs. The control value may represent a compensation stepvalue, an index associated with the required voltage value or an actualvoltage value. The voltage is thus controlled in a stepwise fashion. Inone example system, a SPAD device was tested over a range oftemperatures from −10° C. to 60° C. and the voltage typically moved by 4discrete steps. This was mapped to four corresponding oscillatortrimming steps. For example, +1 trimming step, no trimming step, −1trimming step and −2 trimming step. Because of the asymmetric behavior,the −2 trimming step may be replaced by a −3 trimming step. It should beappreciated that in other embodiments, there may not be a one to onemapping of steps. For example, the oscillator may be trimmed one stepfor every two steps of the voltage control.

The trimming step may be any suitable size and, for example, may be 70kHz.

Thus some embodiments are configured to use the information relating tothe change in the voltage differential applied to the SPADs as a measureof temperature and accordingly trim the oscillator to keep theoscillator frequency within the desired window, thus avoiding problemswith harmonics and the PLL frequency interfering with communication bandfrequencies. In other embodiments, this technique may be used with anyother parameter of the sensing device.

Thus, in some embodiments the voltage in the sensor itself can be useddetermine the temperature. The voltage of the SPAD may be determined bythe sensor itself rather from a temperature sensor input. This mayprovide a relatively coarse resolution for temperature measurement, butin some applications, this may be enough to be used to compensate forthe range drift calculation over temperature

In other embodiments, an external temperature sensor is used for thetemperature measurement. This may give a better temperature measurementand hence may provide a better input for the temperature compensationfor the range output, for some applications.

Reference is made to FIG. 7 which schematically shows a graph ofdetermined range distance versus temperature for a number of devices.The actual range distance is a constant value of 100 mm. The data isnormalized for the reference temperature of 23° C. As can be seen thedetermined range distance is dependent on temperature. The referencetemperature may have any other suitable value in embodiments.

In one embodiment, the gradient of the voltage with respect to thedetermined range may be used. This may be found by determining a seriesof range measurements at the reference temperature of 23° C., andrecording the associated voltage control value VHV. This may be done fora number of different devices.

Gradient of VHV to range=x. In one embodiment, x is 0.82. Correctionthat needs to be applied=−1/x (and where x is 0.82, the correctionis=−1.2.).

In the embodiments, a correction algorithm is applied to the raw rangevalue: New range=Initial range value +1/x* (reference VHV value−actualVHV value)

In some embodiments, the reference VHV value could be obtained byreading the value from memory or form other stored values. Therespective value may be programmed at manufacture, for example. Forexample, a value representative of the maximum VHV range value may beobtained and this may be reduced by a given value or a valuerepresentative of the minimum VHV range value may be obtained and thismay be increased by a given value. These may be values programmed in thedevice at production and correspond to the boundaries of the region 202shown in FIG. 4.

Reference is made to FIG. 8, which shows the data of FIG. 7 which hasbeen corrected using the above equation. As can be seen, there is asmaller deviation between the measured range and the actual range (again100 mm) over a wider range of temperatures.

In another embodiment the following correction algorithm is applied.Instead of applying −1/x factor to VHV, the following is applied

For temperature≥RT (Determined by seeing if the actual VHV≥referencevalue. The reference value may be as discussed previously):

New range=Raw Range+1/x*(reference value−actual VHV)

For temperature<RT (determined by seeing if the actual VHV<referencevalue)

New range=Raw Range+y*(reference value−actual VHV)

In one embodiment y is 3. It should be appreciated the values 1/x and ycan be tuned for different products.

This latter correction algorithm is thus able to take into account thecase that range drift at lower temperatures may be more severe than athigher temperature. In some embodiments, the value of 1/x and y areselected such that the new range value will be within a given toleranceband about the actual value over a given temperature range. Thetemperature range may be any suitable range such as −10° C. to 6° C.

Reference is made to FIG. 9 which show the data of FIG. 7 which has beencorrected using the more complex correction algorithm. As can be seen,the range determined at lower temperatures is more accurate than thatdetermined using the less complex algorithm.

Note the range drift may be a fixed offset that occurs at all distances,hence it cover the full operation range of the sensor.

The correction values are specific to the sensor used and differentvalues may be required by different products. In general, range driftoccurs with temperature changes. This can be regarded as a fixed andknown drift and since the temperature can be measured directly orindirectly, then range drift can be corrected or compensated.

Some embodiments may use other sensors, instead of SPADs. These sensorsmay be integrating elements, rapid charge transfer photodiodes or anyother suitable device which generates events on reception of the lightinformation.

It should be appreciated that the above described arrangements may beimplemented at least partially by an integrated circuit, a chip set, oneor more dies packaged together or in different packages, discretecircuitry or any combination of these options.

It should be appreciated, that an application of some embodiments in aranging device has been described. However, it should be appreciatedthat this is only one example of an application of many differentembodiments. Other embodiments may be used with any other application ofa SPAD or SPAD array or any other suitable photo sensitive device orphoto sensitive device array.

Some embodiments may be provided in a device 500 such as shown in FIG.10. The device 500 may comprise any one of the SPAD or the likearrangements as previously described and referenced 502. An output fromthe SPAD arrangement may be provided to the processor 508. Based on theoutput provided by the processor an information or control signal may beoutput to function block 506. The function block may be a controllerwhich is configured to cause one or more actions in response todetecting a presence of an object. The function block may be a displaywhich is configured to display a measurement result.

It should be appreciated that the device may be any suitable device. Byway of example only and without limitation, the device may be a mobiletelephone, smart phone, tablet, computer, measuring device, switchcontroller such as for a light, controlling a water supply such as in atap or toilet, door controller, distance sensor, impact controller, orany other suitable device.

Various embodiments with different variations have been described above.It should be noted that those skilled in the art may combine variouselements of these various embodiments and variations.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the scope of thepresent invention. Accordingly, the foregoing description is by way ofexample only and is not intended to be limiting. The present inventionis limited only as defined in the following claims and the equivalentsthereto.

What is claimed is:
 1. A device comprising: a single photon avalanchedetector (SPAD); a voltage regulator coupled to the SPAD; a counterhaving an input coupled to an output of the SPAD; and a controllerconfigured to perform a calibration test by: modifying a voltage acrossthe SPAD by controlling the voltage regulator to cycle through aplurality of output voltages; determining, with the counter, a pluralityof counts, wherein each count of the plurality of counts is associatedwith a respective voltage of the plurality of output voltages; selectinga first voltage of the plurality of output voltages based on theplurality of counts; compare the first voltage with a reference voltage;and determining a temperature drift based on the comparison.
 2. Thedevice of claim 1, further comprising an oscillator, wherein thecontroller is further configured to, during the calibration test, adjusta frequency of the oscillator based on the temperature drift.
 3. Thedevice of claim 2, wherein the controller is configured to adjust thefrequency of the oscillator by using a look up table.
 4. The device ofclaim 1, further comprising a non-volatile memory, wherein a valueassociated with the reference voltage is stored in the non-volatilememory.
 5. The device of claim 1, wherein the SPAD comprises: a p-njunction coupled between a first supply node and a first node; atransistor coupled between a second supply node and the first node; andan inverter coupled between the first node and the output of the SPAD.6. The device of claim 5, wherein the voltage regulator is coupled tothe first supply node.
 7. The device of claim 5, wherein the voltageregulator is coupled to the second supply node.
 8. The device of claim1, further comprising a plurality of SPADs, wherein the plurality ofSPADs comprises the SPAD, and wherein the plurality of SPADs are coupledto the counter via an OR-TREE.
 9. The device of claim 1, wherein thecontroller is configured to perform the calibration test atpredetermined time intervals.
 10. The device of claim 1, furthercomprising a light source driver configured to control a light source,wherein the controller is further configured to control the light sourcedriver during the calibration test.
 11. The device of claim 1, wherein:modifying the voltage across the SPAD comprises increasing the voltageacross the SPAD; and selecting the first voltage comprises determining asecond voltage at which the SPAD is firing, wherein the first voltage isequal to the second voltage plus a predetermined voltage.
 12. A methodcomprising: cycling through a plurality of voltage across a singlephoton avalanche detector (SPAD); determining, with a counter coupled tothe SPAD, a plurality of counts, wherein each count of the plurality ofcounts is associated with a respective voltage of the plurality ofvoltages; selecting a first voltage of the plurality of voltages basedon the plurality of counts; comparing the first voltage with a referencevoltage; and determining a temperature drift based on the comparison.13. The method of claim 12, further comprising adjusting a frequency ofan oscillator based on the temperature drift.
 14. The method of claim13, wherein adjusting the frequency of the oscillator comprises using alook up table.
 15. The method of claim 13, wherein adjusting thefrequency of the oscillator comprises adjusting the frequency of theoscillator to avoid harmonics with one or more 4G frequency bands. 16.The method of claim 12, further comprising storing the reference voltageinto non-volatile memory.
 17. The method of claim 12, wherein: cyclingthrough the plurality of voltages comprises increasing the voltageacross the SPAD; and selecting the first voltage comprises determining asecond voltage at which the SPAD is firing, wherein the first voltage isequal to the second voltage plus a predetermined voltage.
 18. The methodof claim 12, further comprises determining a distance to an object bydetermining a first distance, wherein the distance to the object isequal to the first distance plus 1/x times (the reference voltage minusthe first voltage), wherein x is a predetermined constant.
 19. Themethod of claim 18, wherein the distance to the object is equal to thefirst distance plus 1/x times (the reference voltage minus the firstvoltage) when a current temperature is greater than or equal to apredetermined threshold, and wherein the distance to the object is equalto the first distance plus 1/y times (the reference voltage minus thefirst voltage) when a current temperature is lower than thepredetermined threshold, wherein y is a predetermined constant.
 20. Themethod of claim 19, wherein y is greater than x.
 21. A devicecomprising: means for cycling through a plurality of voltage acrossmeans for sensing light; means for determining a plurality of countsreceived from the means for sensing light, wherein each count of theplurality of counts is associated with a respective voltage of theplurality of voltages; means for selecting a first voltage of theplurality of voltages based on the plurality of counts; means forcomparing the first voltage with a reference voltage; and means fordetermining a temperature drift based on the comparison.